Multimode optical fibers are used in many applications, such as communications networks, sensors systems, avionics, and medical instruments. Though the first applications were more related to communications, the multimode fibers are now part of applications where delivering optical power is the prime requirement. As lasers, diodes and laser diode bars grow in power and have improved brightness, multimode fibers are also found more often in industrial lasers applications. In particular, fiber lasers have been improved in their design and are now capable of delivering many hundreds of watts of output. High power fiber lasers are all based on double-clad fibers (DCF). In such fibers, the laser light is transmitted through to double-clad fiber core, whereas the optical power pump light is guided in the first optical cladding of the fiber. A second optical cladding creates the outer waveguide. Because the cladding is larger than the core, greater optical power can be injected in the fiber, providing more pump power to the gain core of the double-clad fiber, thus greater output power for the laser. A general description of such arrangement is found in U.S. Pat. No. 4,829,529 of Kafka. Though pump power and core light can be injected using bulk optics such as lenses, mirrors and dichroic filters, the push of commercialization and industrialization is going towards using optical fiber components to provide coupling into the double-clad fibers. These components are designed to take one or several multimode fibers that are connected to fiber pigtailed laser diodes, laser diode bars or any pump power light sources and to connect them to the cladding of the double-clad fiber.
There are two approaches to coupling pump light into the double-clad fibers. One is to inject light by the end, called end-pumping, the other manages to couple light from the side, called side-pumping.
Many patents propose devices and techniques to achieve end pumping. The simplest is to splice a single multimode fiber with a diameter and a numerical aperture (NA) smaller than that of the DCF. If multiple fibers are required, then a fiber bundle can be fused, tapered and cleaved as disclosed in U.S. Pat. No. 4,392,712 or No. 4,330,170. The tapered fiber bundle (or TFB) half is finally spliced to the DCF as described in U.S. Pat. No. 6,823,117.
Because the bundle is tapered, basic brightness conservation has to be applied between the bundle and the DCF. The tapering of the bundle increases the longitudinal angle AZ of the rays in the multimode structure, but the diameter of the bundle φb is decreased. For the guided rays that have the largest longitudinal angle θz, the numerical aperture of the pump fiber NAb is given by the equation:nco sin θz=NAb where n™ is the refractive index of the core of the pump fibers. The brightness conservation is thus described by the relationship:φbNAb<φDCFNADCF where φDCF is the diameter of the DCF cladding in which the pump has to be injected and
NADCF is the numerical aperture of this cladding. With this relationship, multiple fiber combiners can be made, such as 7×1 (7 multimode fibers into one output fiber) or 19×1. With the proper choice of fiber diameter and numerical aperture, these couplers can converse brightness of the fiber pigtailed pump to the DCF.
However, in double-clad fiber lasers, the power in the core has to output somewhere. With these components, it is only possible to input the double-clad fiber from one end only. For lasers requiring more input fibers or especially for amplifiers, one needs to add a signal fiber in the middle of the bundle to input or output the signals. This complicates the bundle design because it puts constraints on bundle geometry as shown in U.S. Pat. No. 5,864,644 of Di Giovanni and U.S. Pat. No. 6,434,302 of Fidric. The signal fiber is then tapered and certain taper ratios must be met to reduce the splice loss between the signal fiber core in the bundle and the core of the DCF. Because of the geometry, the most common device is a (6+1)×1 combiner (6 pump fibers surrounding 1 signal fiber into the DCF). In this configuration, all the fibers in the middle of the bundle are of the same diameter. When the signal fiber is a large core fiber often few-moded, then tapering is even more restricted as is described in U.S. Patent Application Publication No. 2005/0094952 A1 of Gonthier et al. The same applies for signal fibers that are polarization maintaining fibers. In this case, because the central fiber is not a pump fiber, the loss of brightness from the pump fiber to the DCF will be approximately 15% worse.
Thus, the advantage of end-pumping resides in that the multiple pump fibers can be combined and brightness can mostly be preserved as long as all the pump ports are used. There are restrictions on structure geometries and signal fibers if a signal need-through fiber is required and there are only 2 ends to a DCF.
The second approach, namely side-pumping, can be achieved in different ways, but they are all somewhat related to the fused couplers first disclosed in U.S. Pat. No. 4,291,940 by Kawasaki et al. describing biconical tapered fiber couplers. When two or more multimode fibers are fused longitudinally and tapered, the light escapes from one fiber because the longitudinal angles of the modes increase in the down-tapering section and become coupled to the other multimode fibers. As the diameter increases again in the up-tapering output section, the longitudinal angles of the modes decrease to a value below the numerical aperture of the output fiber, creating a low-loss fiber optic component. Such simple devices can easily couple light into a DCF but they tend to produce a uniform power distribution in the multimode waveguides and thus lots of power remains in the multimode pump fibers. Such coupling can be optimized however as described in U.S. Pat. No. 6,434,295 by MacCormack et al. In a simplified coupling model, one can assume that the coupling or power distribution in a multimode fused biconical taper coupler is proportional to the relative area of the fibers fused in the coupler. Thus, coupling a pump fiber and a DCF fiber where the two fibers have the same diameter will result in a 50% coupling of the pump light. If one fabricates a coupler coupling two DCF fibers with one pump fiber of the same diameter, then 66% of the pump fiber light is transferred into the DCF fibers. Furthermore, MacCormack proposes to increase that coupling by making the coupler transversely asymmetric. If the numerical aperture of the pump fiber is smaller than the numerical aperture of the DCF fiber, then the pump fiber can be tapered proportionally to the ratio of the numerical aperture, as per the conservation of brightness rule. The coupler is then fused in this asymmetrical region where the ratio of the area is now in favour of the DCF. As an example, if the pump fiber has a numerical aperture of 0.22 and the DCF of 0.44, the pump fiber can be tapered by a factor of 2, its area is thus reduced by a factor of 4. The ratio of the area between the two fibers goes from 50%/50% in the case of a untapered pump fiber to 20%/80% for the asymmetric coupler with the tapered pump fiber, thus coupling now 80% of the pump light into the DCF. This technique unfortunately is not very efficient as per use of brightness because of the power remaining in the pump fiber and in order to get very good coupling efficiency, one requires the greatest difference in the area between the pump and the DCF and the brightness loss is also directly proportional to this ratio. Thus, the better is the coupling, the worse the brightness.
Another way of creating asymmetry in the coupling ratio is proposed in U.S. Pat. No. 4,586,784. It is also based on fusing fibers longitudinally together, but is now using a longitudinal tapering in the pump fiber that is fused to the other multimode fibers, to create a larger asymmetry in the coupling. In U.S. Pat. No. 5,999,673 there is also proposed such a taper arrangement fused to a DCF fiber, but in this case a single pump fiber is tapered to a very small diameter, by a factor much greater than 2. This causes an increase in the angle of the rays propagating in the pump fiber. However, because the taper is fused to the DCF, light starts to escape for the pump fiber into the DCF before the rays reach an angle where they would not longer be guided by the DCF. To insure this, the launch condition of the laser diode to the pump fiber is controlled so that the longitudinal angle of the rays coupled from the laser do not exceed that of the DCF, even after these rays have been through the tapered portion fused to the coupler. The relationship between the angle of the laser pigtail and the critical angle of the DCF is given as the square root of the ratio of the sum of the areas of the non-tapered pump fiber and the DCF fiber over the output DCF fiber area, which assumes that the fiber is tapered to a negligible diameter. This technique has the advantage of producing coupling efficiency close to 100%, however, its configuration does not optimize brightness. Using its relation, the relative loss of brightness is equal to the ratio of the divergence angle and the critical angle of the DCF. For two fibers of the same diameter, this gives 40% which is worse than for a (6+1)×1 combiner. Furthermore, the pump source will tend to fill the numerical aperture of the pump fiber if a moderate length of pump fiber is between the laser and the coupler, thus changing the divergence angle. of the rays in the pump fiber. This will result in loss at the output of the coupler because of the mismatch with the DCF.
Thus, the advantages of side-pumping are that the signal fibers are always there continuous, they can be cascaded one after the other to increase the amount of power coupled, and they do not suffer the geometry restriction of end-pump combiner. Its disadvantage is that they are much less efficient in brightness conservation. This has a direct impact on the length of the amplifier and on a laser cavity length because to couple a pump source of a given pigtailed pump fiber, requires a larger diameter fiber and thus longer gain fiber because the absorption of the gain media is worse as diameter grows. Furthermore, the use of a single pump fiber is limiting the flexibility of the design configuration if several pumps are required.
There is thus a need to provide an improved coupling approach to inject pump fiber optical power into a DCF fiber while optimizing both the advantages of side-pumping of a continuous DCF with the better brightness efficiency of end-pump combiners.